It has been hitherto known that hollow polymer particles cause irregular reflection of irradiated light due to the presence of a void in the interior thereof, and thus, when the hollow polymer particles are coated on a material, whiteness of the material is enhanced and the material is made opaque.
It is considered that, as the diameter of the hollow polymer particles becomes large, the opacifying effect becomes prominent.
Polymer particles having a special shape, other than the hollow polymer particles, also are known as exhibiting an opacifying effect. For example, in Japanese Unexamined Patent Publication (hereinafter abbreviated to "JP-A") No. H5-222108, polymer particles having at least one through-hole extending from the particle surface to the interior thereof, which is formed when the particles are dried, and a process for producing the polymers are described. It is described therein that the polymer particles are usable as a coating composition and an organic pigment for paper coating color. However, the particle size of the polymer particles, produced by this process, are small, i.e., at most about 0.2 .mu.m, and thus, the opacifying effect of the pigment is poor.
A thermal recording material has been proposed in JP-A H2-164580, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains porous and non-spherical hollow polymer particles having undulations on the surface which are formed by allowing hollow polymer particles to shrink. These non-spherical hollow polymer particles exhibit good thermal response, but have poor adhesion between the thermal recording layer and the support.
Another thermal recording material has been proposed in JP-A H5-573, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains spherical or ellipsoidal plastic hollow particles. The plastic hollow particles are fine foamed plastic hollow particles having an average particle diameter of 2.0 to 20 .mu.m, preferably 3 to 10 .mu.m. It is however difficult to prepare fine foamed plastic hollow particles which are uniform and have a narrow particle size distribution.
A further thermal recording material has been proposed in JP-A H2-57382, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains fine hollow polymer particles having a particle diameter of not larger than 5 .mu.m, preferably 0.1 to 3 .mu.m. These hollow polymer particles exhibit a poor effect for enhancement of thermal response, and further have problems of causing sticking and deposition of tailings on a thermal head.
Emulsified vinyl polymer particles have been proposed in JP-A-H2-14222, which have a flat shape, and at least one major surface of which is concave. It is described therein that these polymer particles are used as an additive for a paint, a paper coating color, or a coating color for information recording paper and other materials. It is however indispensable for the preparation of the polymer particles to conduct emulsion polymerization using a non-aqueous organic solvent such as an aliphatic hydrocarbon, and, after the completion of polymerization, to remove the organic solvent from the reaction system. Due to the flat shape, the polymer particles exhibit poor opacifying effect, and, where they are used for a thermal recording paper, they exhibit a poor heat-insulating effect.